In medical radiography an image of a patient's tissue and bone structure is produced by exposing the patient to X-radiation and recording the pattern of penetrating X-radiation using a radiographic element containing at least one radiation-sensitive silver halide emulsion layer coated on a transparent (usually blue tinted) film support. The X-radiation can be directly recorded by the emulsion layer where only limited areas of exposure are required, as in dental imaging and the imaging of body extremities. However, a more efficient approach, which greatly reduces X-radiation exposures, is to employ an intensifying screen in combination with the radiographic element. The intensifying screen absorbs X-radiation and emits longer wavelength electromagnetic radiation which silver halide emulsions more readily absorb. Another technique for reducing patient exposure is to coat two silver halide emulsion layers on opposite sides of the film support to form a "double coated" radiographic element.
Diagnostic needs can be satisfied at the lowest patient X-radiation exposure levels by employing a double coated radiographic element in combination with a pair of intensifying screens. The silver halide emulsion layer unit on each side of the support directly absorbs about 1 to 2 percent of incident X-radiation. The front screen, the screen nearest the X-radiation source, absorbs a much higher percentage of X-radiation, but still transmits sufficient X-radiation to expose the back screen, the screen farthest from the X-radiation source. In the overwhelming majority of application the front and back screens are balanced so that each absorbs about the same proportion of the total X-radiation. However a few variations have been reported from time to time. A specific example of balancing front and back screens to maximize image sharpness is provided by Luckey, et al., U.S. Pat. No. 4,710,637. Lyons et al. U.S. Pat. No. 4,707,435 discloses in Example 10 the combination of two proprietary screens, Trimax 2TM employed as the front screen and Trimax 12FTM employed as a back screen. Rossman and Sanderson, "Validity of the Modulation Transfer Function of Radiographic Screen-Film Systems Measured by the Slit Method", Phys. Med. Biol., 1968, vol. 13, pp. 259-268, report the use of unsymmetrical screenfilm assemblies in which either the two screens had measurably different optical characteristics or the two emulsions had measurably different optical properties.
An imagewise exposed double coated radiographic element contains a latent image in each of the two silver halide emulsion units on opposite sides of the film support. Processing converts the latent images to silver images and concurrently fixes out undeveloped silver halide, rendering the film light insensitive and transparent. When the film is mounted on an illuminated viewer, the two superimposed silver images on opposite sides of the transparent support are seen as a single image. against a white, illuminated background.
An art recognized difficulty with employing double coated radiographic elements in combination with intensifying screens as described above is that some light emitted by each screen passes through the transparent film support to expose the silver halide emulsion layer unit on the opposite side of the support to light. The light emitted by a screen that exposes the emulsion layer unit on the opposite side of the support reduces image sharpness. The effect is referred to in the art as crossover.
The most successful approach to crossover reduction yet realized by the art, consistent with viewing the superimposed silver images through a transparent film support without manual registration of images, has been to employ double-coated radiographic elements containing spectrally sensitized tabular grain emulsions of high aspect ratio or intermediate aspect ratio, illustrated by Abbott et al. U.S. Pat. Nos. 4,425,425 and 4,425,426, respectively. Whereas radiographic elements prior to Abbott et al. typically exhibited crossover levels of at least 25 per cent, Abbott et al. provide examples of crossover reductions in the 15 to 22 per cent range.
More recently, Dickerson et al. U.S. Pat. No. 4,803,150 demonstrated that by combining the teachings of Abbott et al. with a processing solution decolorizable microcrystalline dye located between at least one of the emulsion layer units and the transparent film support, "zero" crossover levels can be realized. Dickerson et al. U.S. Pat. No. 4,900,652 adds to these teachings a specific selection of hydrophilic colloid coating coverages in the emulsion and dye containing layers to allow the "zero" crossover radiographic elements to emerge dry to the touch from a conventional rapid access processor in less than 90 seconds with the crossover reducing microcrystalline dye decolorized.
By minimizing the effects of crossover it became feasible to prepare double coated elements in which the emulsions on the opposite sides of the support have different sensitometry. Dickerson and Bunch U.S. Pat. Nos. 4,994,355 and 4,997,750 disclosed "zero" crossover, double coated radiographic elements in which the emulsion layer units on opposite sides of the support differ, respectively, in contrast and in speed. Dickerson and Bunch U.S. Ser. No.502,153, filed Mar. 29, 1990, now U.S. Pat. No. 5,108,881, disclosed zero crossover double coated radiographic elements in which the emulsion layer units on opposite sides of the support differ in contrast in a manner particularly suited to permitting flexibility in the choice of intensifying screen pairs employed.
Bunch and Dickerson U.S. Pat. No. 5,021,327 disclosed zero crossover double coated radiographic elements in combination with a pair of intensifying screens, where the combination of the back emulsion layer unit and its intensifying screen exhibits a photicity twice that of the combination of the front emulsion layer unit and its intensifying screen, where photicity is the product of screen emission and emulsion layer unit sensitivity. All of the elements just described can be referred to as sensitometrically asymmetrical.
These combinations of asymmetrically coated radiographic elements used with different screens present a practical problem with their use in the darkrooms of typical radiological laboratories. In practice, for each radiograph taken of a patient, the film, i.e., the photographic element, is typically removed from a package in darkness or under dim, dark red safelights and loaded into a hinged, light-tight cassette. The screens are mounted on the inside of the two hinged sides of the cassette so that they are positioned in close contact with the inserted film when the cassette is closed. When an asymmetrically coated film is used in a cassette with two different screens, the film must be oriented in the proper position in order to achieve the desired sensitometry. Since the film looks identical on both sides under the dim lighting conditions of the darkroom, the technician has no certain way of determining which side of the film should eventually face the source of the X-radiation unless it is marked is some way. The front of the closed cassette is loaded into the exposure device with a labeled side facing the X-ray source. After the radiograph is taken, the film is removed from the cassette for processing and the cassette is reloaded for another radiograph.
Jebo et al., U.S. Ser. No. 502,341, filed Mar. 29, 1990, now Statutory Invention Registration H1105, described various orienting means, all mechanical or electrical in nature, for properly positioning an asymmetric radiographic element into the cassette. The means described all involved designing the cassette and film assembly in such a way that the film will only fit in the cassette in a single orientation. The means also involved marking the film and/or the screens so that they can be aligned in an obvious way while loading under dark or safelight conditions. They relied on means such as corner cuts, corner marks, and dimples in the film, alignment of holes for insertion of a pin, and of electrical contacts, etc., to prevent misalignment of the asymmetric film in the cassette.